General Synthetic Routes of Metallic Nanoparticles

The goal in preparing spherical gold nanoparticles is to produce a population of narrowly dispersed gold cores passivated and stabilized by an organic ligand shell. Most commonly, Au (III) salt is reduced to Au (0) to form an activated species, either in a single step or via a Au (I) intermediate followed by reduction to Au (0).

These activated Au (0) species are thermodynamically unstable and rapidly aggregate to form nuclei and eventually the desired gold nanoparticle, as presented in Fig. (1.9). the most commonly referenced mechanism for this process comes from LaMer theory (LaMer et al.

, 1950).

Figure (1.9): Schematic diagram of gold nanoparticle growth.

LaMer theory is broken into three stages (Fig. (1.9)), (I) precursor addition / formation of the activated species, (II) nucleation and (III) core growth. In Stage I, the precursor [Au (III)] is chemically reduced to form the activated Au (0) species. As the concentration of this activated species increases, the system passes into the supersaturated regime (Stage II) at which the Au (0) species begin to aggregate and form nuclei.

Get quality help now

Dr. Karlyna PhD

Verified writer

Proficient in: Chemistry

4.7 (235)

“ Amazing writer! I am really satisfied with her work. An excellent price as well. ”

+84 relevant experts are online

Hire writer

Nucleation continues until the concentration of Au (0) in solution drops below the supersaturated regime (Stage III), at which the formed nuclei continue to grow through diffusion. This process typically occurs in the presence of organic ligands which associate with the surface and eventually passivate it. Although some syntheses have been developed for bare gold particles, these species are typically thermodynamically unstable and acmulate to bulk metal.

Methods for synthesize of metallic nanoparticles

Nanoparticles can be synthesized using a variety of methods including physical, chemical, biological, and hybrid techniques (Mohanpuria et al.

,2008) (figure-1.10). Methods employed for the synthesis of nanoparticles are broadly classified under two processes such as "Top-down" process and "Bottom-up" process (Sepeur et al., 2008). (figure-1.10).

Top-down approach: Bulk material is broken down into particles at nanoscale with various lithographic techniques e.g.: grinding, milling etc. Bottom-up approach: Atoms self-assemble to new nuclei which grow into a particle of nanoscale.

The production of nanoparticles through conventional physical and chemical methods results in toxic byproducts that are environmental hazards. Additionally, these particles cannot be used in medicine due to health-related issues, especially in clinical fields (UK, et al., 2009 and ParasharR et la., 2009)

Figure (1.10): Different approaches and methods for synthesizing nanoparticles

Chemical Methods of preparation

In this approach, initially the nanostructured building blocks (ie, nanoparticles) are formed and, subsequently, assembled into the final material using chemical or biological procedure(s) for synthesis. A distinct advantage of the bottom-up approach is the enhanced possibility of obtaining metallic nanoparticles with comparatively lesser defects and more homogeneous chemical composition(s).

In the top-down approach, a suitable starting material is reduced in size using physical (eg, mechanical) or chemical means. A major draw back of the top-down approach is the imperfection of the surface structure. Such defects in the surface structure can have a significant impact on physical properties and surface chemistry of the metallic nanoparticles due to the high aspect ratio.

The traditional and most widely used methods for synthesis of metallic nanoparticles use wet-chemical procedures. A typical procedure involves growing nanoparticles in a liquid medium containing various reactants, in particular reducing agents (eg, sodium borohydride (Kim et al., 2007) or potassium bitartrate (Tan et al., 2003) or methoxypolyethylene glycol (Mallick et al., 2004).or hydrazine (Li et al., 1999).

Sodium citrate is the most common one for gold nanoparticles Gold chloroauric asid salt (H [AuCl4]) was used as gold salt in the experiments. Trisodium citrate (Na3C6H5O7.2H2O) was used as reducing agent (Turkevich et al. 1951) . Gold salt was mixed and boiled to the boiling point (97.5°C) at prepared concentration to start the synthesis reaction.

After adding the prepared sodium citrate to the solution, sodium citrate turned to citric acid. At that stage yellow coloured solution suddenly became transparent and colourless. It changed to black and after than slowly to wine red (Shipway et al., 2000). Gold salt synthesis was concluded at this point as following

2HAuCl4 + 3C6H8O7 (citric acid) ? 2Au0

+ 3C5H6O5 (3-ketoglutaric acid) + 8HCl + 3CO2

Figure ( 1.11): Scheme for AuNP synthesis using the Turkevich method

It was reported that nearly monodispers particles were synthesized by citrate reduction while particle size was controlled by initial reagent concentration (Hostettler et al., 1998).

Many chemical methods are used where detergents such as sodium dodecyl benzyl sulfate or polymer such as polyvinyl pyrrolidone (Kim et al., 2007) are used to stabilize the colloidal nanoparticles. Generally, the chemical methods are low-cost for high volume; however, their drawbacks include contamination from precursor chemicals, use of toxic solvents, and generation of hazardous by-products (Faraday 1857)

• Phase transfer of Au(III) ions from aqueous layer to organic layer (i.e. toluene or benzene) with the help of phase transfer agent TOAB (tetrabutylammonium bromide); HAuCl4(aq) +TOAB(toluene)?TOA+AuCl4-(toluene)
• Reduction of Au (III) to Au (I) by the added thiols;
• Reduction of Au(I) to Au(0) by NaBH4 in the presence of thiols and/or other sulfur containing ligands that leads to the formation of AuNPs (Li, 2011; Zhao et al., 2013). Also, the reduction by borohydride (NaBH4) has been used for a many years (Wagner et al., 2008; Kim et al., 2007; Schlesinger et al., 1953).

Different wet chemical methods have been used for the synthesis of metallic nanoparticles dispersions, the most common involving the use of reducing agents such as sodium borohydride (Kim et al., 2007), potassium bitartrate (Tan et al., 2003), hydrazine N2H4 (Li et al., 1999), methoxypolyethylene glycol (Mallick et al., 2004), dry ethanol (Ayyanppan et al., 1997), polyethylene glycol PEG (Longenberger et al., 1995),

reducing sugars (Panigrahi et al., 2004),stannous chloride (Vaskelis et al., 2007) and ascorbic acid (Wagner and K?hler, 2005), amine or hydroxyl-containing molecules such as branched poly(ethyleneimine) (Note et al., 2006), alginate (Yang and Pan, 2012; Balavandy et al., 2015), amino acid (Selvakannan et al., 2004) or chitosan (Shih et al. 2009) were also reported as a suitable reducing agents for metallic nanoparticles preparation.

There are also several modified chemical methods including seed-mediated growth where small particles (produced by other techniques like irradiation) are utilized as seeds and then fresh metallic ions are reduced by a reducing agent and grow along the surface of the seed particle (Samanta et al., 2010).

Bottom up synthesis techniques employ an agent to stop growth of the particle at the nanoscale (Kamat, 2002). Capping materials (e.g surfactant, charged molecules or polymer) such as sodium dodecyl benzyl sulfate (Li et al., 1999), polyvinyl pyrrolidone (Tan et al., 2003), cetyltrimethylammonium bromide or sodium dodecyl sulfate (Tu et al., 2010) are added to the reaction mixture to prevent aggregation and precipitation of the metal nanoparticles out of the solution.

Choice of the reduction technique, time, and capping material determines the size and shape of the nanoparticles generated. (Eustis and El-Sayed, 2006). Generally, the chemical methods are low-cost for high volume; however, their drawbacks include contamination from precursor chemicals, use of toxic solvents, and generation of dangerous and hazardous reaction byproducts.

Green chemical methods are based on the use of pure chemical reagents which are green and environmentally friendly. In most of these green methods, carbohydrates such as glucose (Raveendran et al., 2003; Panigrahi et al., 2004), sucrose (Panigrahi et al., 2004), starch (Raveendran et al., 2003; Vigneshwaran et al., 2006), chitosan (Sun et al., 2008), and calcium alginate (Saha et al., 2009) are used as reducing agents, capping agents, or both.

Also, biodegradable and nontoxic polymers such as carboxy methyl cellulose (CMC) and polyethylene glycol (PEG) (Virkutyte and Varma, 2011) have been successfully employed for the green synthesis of metallic nanoparticles. The nanoparticles produced by these methods are generally spherical.